The present invention relates to a method for preparing dinitrotoluene.
More precisely, the invention relates to an improved method for preparing dinitrotoluene from mononitrotoluene.
Dinitrotoluene is a significant industrial product since it is the production intermediate of toluene diisocyanate.
Dinitrotoluene is obtained by means of double nitration of toluene. Since the nitration operations are not selective, dinitrotoluene is an admixture of isomers of 2,4-dinitrotoluene and 2,6-dinitrotoluene associated with different impurities which are other isomers such as 2,3-dinitrotoluene, 2,5-dinitrotoluene, 3,4-dinitrotoluene and 3,5-dinitrotoluene. In the admixture, the ratio between the 2,4-dinitrotoluene and the 2,6-dinitrotoluene is equal to 4±0.3 and the content of impurities is generally between 3 and 4.5% by weight.
EP-A 0 903 336 describes a method for preparing dinitrotoluene by means of double nitration of toluene using an admixture of nitric acid and sulphuric acid with the characteristic of using dilute sulphuric acid.
Conventionally, the production of dinitrotoluene is carried out in accordance with a method in two steps as described in particular in the encyclopaedia KIRK-OTHMER, Encyclopedia of Chemical Technology, 15, p. 927 ff. (3rd edition) and by H. Hermann et al. in the work Nitration, Recent Laboratory and Industrial Developments, (Chapter 21, Lyle F. Albright, Richard V. C. Carr, Robert J. Schmitt, American Chemical Society, Washington D.C. 1996).
The first step involves carrying out the nitration of toluene with nitric acid, in the presence of sulphuric acid, and separating the mononitrotoluene obtained from the reaction admixture followed by a second step of nitration of the mononitrotoluene obtained using nitric acid and in the presence of sulphuric acid followed by separation of the dinitrotoluene obtained.
There is therefore carried out, in a first step, the nitration of the toluene using a nitrating admixture whose composition may vary and comprise, for example, from 50 to 60% by weight of sulphuric acid, from 15 to 40% by weight of nitric acid and from 10 to 20% by weight of water.
The quantity of nitric acid used is generally greater than the quantity required by the stoichiometry of the nitration reaction, the molar ratio of nitric acid/toluene being between 1.05 and 1.2.
After the reaction is complete, there is separation of an aqueous phase comprising the residual acids (sulphuric acid, nitric acid) with mainly sulphuric acid and an organic phase comprising mainly mononitrotoluene.
There is carried out in a following step the nitration of the mononitrotoluene included in the organic phase using a nitrating admixture.
Generally, it is advantageous, for reasons of productivity and equipment size, to begin with a nitrating admixture with a low water content. In this manner, an admixture is preferably used comprising from 55 to 70% by weight of sulphuric acid, from 20 to 44.5% of nitric acid and from 0.5 to 10% by weight of water.
The quantity of nitric acid used in this nitration reaction is greater than the quantity required by the stoichiometry of the reaction, the molar ratio of nitric acid/mononitrotoluene being between 1.05 and 1.2.
At the end of the reaction, there are obtained an aqueous phase and an organic phase which must be separated.
Under the conditions of a nitrating flow which is concentrated in terms of acids, the weight ratio, at the end of the dinitration reaction, between the aqueous phase and the organic phase is less than 1 and is more particularly between 0.75 and 0.95 which signifies that there is very preferably obtained an aqueous phase dispersed in the organic phase.
As mentioned above, it is important to use a nitrating admixture which is concentrated in terms of nitric and sulphuric acid.
Under these conditions, however, it has been found that separation of the organic and aqueous phases carried out in accordance with the technique of separation by decantation was laborious. In this manner, time periods of 48 hours were required in order to carry out the separation.
Furthermore, it has been shown that, since the technique of separation was found to be difficult, it was not possible to reduce the quantity of supernatant nitric acid used during the second nitration operation since, in this instance, the separation was made even more difficult even when the separation was carried out by means of centrifuging.
An object of the present invention is to provide an improved method for nitration of mononitrotoluene.
One of the objects of the invention is to provide a method which allows easier separation of the aqueous and organic phases following the operation for nitration of the mononitrotoluene.
Another object of the invention is to allow the quantity of supernatant nitric acid used in the second nitration step to be reduced and thus to reduce the quantity of nitric acid present in the dinitrotoluene subsequently sent for purification by means of washing: the supernatant nitric acid will then be found in the aqueous effluents of the method which may be recycled or processed.